1. Field of the Invention
The present invention relates to a technique of reading a trimming data used for trimming circuit characteristics. In particular, the present invention relates to a technique of reading the trimming data stored in a nonvolatile memory.
2. Description of Related Art
A nonvolatile memory such as a flash memory and an EEPROM (Electrically Erasable and Programmable Read Only Memory) is known. A memory cell of such a nonvolatile memory has a charge storage layer such as a floating gate. Data writing/erasing is performed by injecting electrons into the charge storage layer or drawing electrons out of the charge storage layer. A relatively high voltage is required in the data writing/erasing, and the high voltage is generated by a charge pump or the like. A reference voltage, which is used for generating the high voltage, is likely to vary with respect to each chip due to manufacturing variability. Also, at a time of data reading, a data stored in a read-target cell is sensed by comparing a cell current flowed from the read-target cell with a predetermined reference current. The reference current also is likely to vary with respect to each chip due to manufacturing variability.
As described above, the manufacturing variability between chips causes the variations of the output voltage of the charge pump and the reference current that is referred to at the time of data reading, which consequently leads to variations of write characteristics and read characteristics. In order to suppress the characteristic variations, it is necessary after manufacture of chips to fine-tune the output voltage of the charge pump and the reference current with respect to each chip. The fine-tuning is called “trimming”.
As a result of the trimming, the output voltage of the charge pump and the reference current are respectively set to optimum values. A data indicating the optimum setting is referred to as a “trimming data” hereinafter. The trimming data is determined with respect to each chip at a test stage after the manufacture of chips, and the determined trimming data is stored in a predetermined memory region. At the power-on, the trimming data is read from the predetermined memory region, and the trimming of circuit characteristics is performed by using the read trimming data. That is, the output voltage of the charge pump and the reference current are respectively adjusted (trimmed) to the optimum values by using the read trimming data.
The trimming data related to the circuit characteristics of the nonvolatile memory may be stored in a memory region of the nonvolatile memory itself. Such a memory region for use in storing the trimming data is referred to as an “EXTRA region” hereinafter. Data types and access addresses for the EXTRA region are fixed, and data once stored in the EXTRA region are not rewritten. At the power-on, the trimming data is first read from the EXTRA region, and then the trimming of circuit characteristics is performed based on the read trimming data.
Note that the reading of the trimming data from the EXTRA region is naturally prior to the trimming of the circuit characteristics. That is to say, the trimming data is not necessarily read out under the optimum circuit characteristics. In other words, read accuracy of the trimming data itself is not necessarily high.
Japanese Laid-Open Patent Application JP-2008-47209 discloses a technique related to the trimming. According to the technique, the same trimming data is stored in two trimming redundancy information storage memory regions. At the power-on, the trimming data are respectively read from the two trimming redundancy information storage memory regions. Then, a sequence circuit determines validity of the read trimming data based on a check sum method or the like. If a positive result is obtained, the read trimming data is adopted. Consequently, the read accuracy of the trimming data is improved.
The inventor of the present application has recognized the following points. A problem peculiar to data read from the nonvolatile memory is “read disturb”. The read disturb means that weak data writing is caused every data reading and thereby data characteristics of a memory cell are varied. In the worst case, a stored data in the memory cell is changed.
As described above, the trimming data is essential for obtaining the optimum circuit characteristics and is always read from the EXTRA region at every power-on procedure. Therefore, it can be said that the trimming data (EXTRA region) is susceptible to the read disturb. The change in data characteristics of the EXTRA region caused by the read disturb leads to read errors such as erroneous reading of the trimming data, which consequently deteriorates the circuit characteristics.
In order to suppress the erroneous reading of the trimming data, for example, a difference (margin) between the cell current and the reference current may be expanded in consideration of variation of the cell current caused by the read disturb. To that end, it is necessary to design a circuit configuration such that a larger cell current can flow. However, this causes increase in a circuit size and power consumption.